The methods for manufacturing an iron-bearing pellet or an iron-bearing briquette (hereinafter referred to as "iron-bearing pellet") include the firing method and the non-firing method from a green iron-bearing pellet or a green iron-bearing briquette (hereinafter referred to as "green iron-bearing pellet") prepared by forming into a granular or solid form a mixture obtained through addition of a binder to main raw materials which comprise at least one of an iron ore fine and a dust mainly comprising iron oxides.
The non-firing method for manufacturing an iron-bearing pellet comprises continuously supplying a green iron-bearing pellet containing a hydraulic binder and water into a treating furnace, blowing a gas at a temperature of for example 100.degree. C. into this treating furnace to heat this green pellet to a target temperature, and holding this green pellet at the target temperature for a prescribed period of time to harden the green pellet. Therefore this manufacturing method is more advantageous than the method for manufacturing an iron-bearing pellet through firing of a green iron-bearing pellet at a high temperature of over 1,000.degree. C. in that it requires a smaller consumption of energy and simpler operations.
In the heat balance in the case where a green iron-bearing pellet supplied to a treating furnace is heated through blowing of a gas at a prescribed temperature when disregarding the thermal loss occurring in the furnace, the heat absorbed by the green pellet is equal to the heat released by the gas blown to the green pellet, as expressed in the following equation (1): ##EQU1## where, C.sub.s : specific heat of the green iron-bearing pellet (Kcal/kg..degree.C.);
M.sub.s : mass of the green iron-bearing pellet (kg); PA1 T.sub.so : charging temperature of the green iron-bearing pellet (.degree.C.); PA1 T.sub.sl : heating temperature of the green iron-bearing pellet (.degree.C.); PA1 C.sub.g : specific heat of the gas (Kcal/Nm.sup.3..degree.C.); PA1 F: flow rate of the gas (Nm.sup.3 /Hr); PA1 T.sub.go : blowing temperature of the gas (.degree.C.); PA1 T.sub.gl : discharge temperature of the gas (.degree.C.); PA1 .tau.: staying time of the green iron-bearing pellet in the furnace (Hr); and, ##EQU2## integral value of the function C.sub.g F(T.sub.go -T.sub.gl) in a time interval of from start of heating to .tau., and when the above function is any constant, the above integral value become the product of the above function and .tau..
The left-hand side of the above-mentioned equation (1) is dependent solely on the quantity of green iron-bearing pellet to be treated and the heating temperature, and irrespective of the heating method of green iron-bearing pellet. It is therefore necessary to set the right-hand side of the equation (1) at a value equal to that of the left-hand side. In this case, in order to minimize the gas flow rate F or the integral value thereof F.sub..tau., it is necessary to increase the gas blowing temperature T.sub.go, whereas, in order to reduce the gas blowing temperature T.sub.go, it is necessary to increase the gas flow rate F or the integral value thereof F.sub..tau.. More particularly, in order to reduce the flow rate of the gas for heating the green iron-bearing pellet supplied to the treating furnace, it is necessary to blow the gas at a temperature higher than the heating target temperature of the green iron-bearing pellet. When the gas blowing temperature cannot be increased, on the other hand, it is necessary to increase the gas flow rate.
A method for manufacturing a non-fired iron-bearing pellet in a short time, in which a green iron-bearing pellet supplied in a treating furnace is subjected to a non-firing treatment, is disclosed in the Japanese Patent Publication No. 29688/72 dated Aug. 3, 1972 (hereinafter referred to as a "prior art").
The prior art discloses the method comprising: supplying a green iron-bearing pellet into a treating furnace, and blowing a steam at a temperature ranging from 160.degree. to 230.degree. C. into said treating furnace to heat said green iron-bearing pellet and thereby hardening the same to manufacture a non-fired iron-bearing pellet.
However, in the prior art, since the temperature of the gas (steam) to be blown into the treating furnace is increased to over the heating target temperature of the green iron-bearing pellet, overheating occurs in the part of the above-mentioned green iron-bearing pellet first brought into contact with this gas, and the water contained in this part is evaporated. As a result, this green iron-bearing pellet does not give a proper hydration reaction, resulting in a lower product quality.
On the other hand, a higher flow rate of the gas blown into the treating furnace leads to a higher manufacturing cost. When, for example, one ton of a green iron-bearing pellet of a temperature of 30.degree. C. is heated by blowing a gas to increase the temperature of the pellet by 60.degree. C. to a heating target temperature of 90.degree. C. of the pellet on condition that an initial temperature of the blown gas is set at 100.degree. C. which is higher by 10.degree. C. than the target temperature and a discharge temperature of the blown gas after heating the green iron-bearing pellet is set at 90.degree. C., the quantity of the gas required for heating the green iron-bearing pellet to said target temperature would be so large as 3,000 Nm.sup.3 per ton of the green iron-bearing pellet as follows as calculated by the above-mentioned equation (1): ##EQU3##
Under such circumstances, when manufacturing a non-fired iron-bearing pellet hardened through non-firing treatment of a green iron-bearing pellet, there is a strong demand for developing a method and an apparatus which permit manufacture of a high-quality non-fired iron-bearing pellet in a short time at a low cost, but such a method and an apparatus are not as yet proposed.